ECHINODERMATA. 



463 



tentacular outgrowths of the water-vascular ring (fig. 258), and possi- 

 bly also forms the nervous system. 



The opening of the cesophageal invagination is at first behind the 

 foremost ciliated ring, but eventually comes to lie in front of it, and 

 assumes a nearly terminal though slightly ventral position (fig. 258). 

 No account has been given of the process by which this takes place, 

 but the mouth is stated by Metschnikoff (though Muller differs from 

 him on this point) to remain open throughout. The further changes 

 in the metamorphosis are not considerable. The ciliated bands dis- 

 appear, and a calcareous ring of ten pieces, five ambulacral and five 

 interambulacral, is formed round the oesophagus. A provisional 

 calcareous skeleton is also developed. 



All the embryonic systems of organs pass in this case directly into 

 those of the adult. 



The metamorphosis of most Holothuroidea is similar to that just de- 

 scribed. In Cucum.aria (Selenka) there is however no Auricularia stage, and 

 the uniformly ciliated stage is succeeded by one with five transverse bands of 

 cilia, and a prae-oral and an anal ciliated cap. The mouth is at first situated 



A 



FIG. 257. THREE STAGES IN THE DEVELOPMENT OF SYNAPTA. A and B are viewed 

 from the ventral surface, and C from the side. (After Metschnikoff.) 



m. mouth; oe. oesophagus; pv. walls of the perivisceral cavity ; wv. longitudinal 

 vessel of the water-vascular system; p. dorsal pore of water-vascular system; cr. 

 ciliated ring formed round the mouth from parts of the primitive ciliated band. 



ventrally behind the prae-oral cap of cilia, but the prse-oral cap becomes 

 gradually absorbed, and the mouth assumes a terminal position. 



In Psolinus (Kowalevsky) there is no embryonic ciliated stage, and the 

 adult condition is attained without even a metamorphosis. There appear 



